Techniques for additive fabrication process optimization and related systems and methods

ABSTRACT

According to some aspects, a method of additive fabrication is provided wherein a plurality of layers of material are formed on a surface of a build platform, each layer of material being formed so as to contact a container in addition to the surface of the build platform and/or a previously formed layer of material. The method comprises forming a layer of material in contact with the container and in contact with a previously formed layer of material, and subsequent to the forming of the layer of the material, separating the layer of material from the container by simultaneously applying a first force to the layer of the material in a direction perpendicular to the surface of the build platform, and a second force to the layer of the material in a direction parallel to the surface of the build platform.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Patent Application No. 62/211,352, filed Aug. 28, 2015,titled “Techniques for Additive Fabrication Process Optimization andRelated Systems and Methods,” which is hereby incorporated by referencein its entirety.

BACKGROUND

Additive fabrication, e.g., 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography, selective or fuseddeposition modeling, direct composite manufacturing, laminated objectmanufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build parts by forming successive layers, which are typicallycross-sections of the desired object. Typically each layer is formedsuch that it adheres to either a previously formed layer or a substrateupon which the object is built.

In one approach to additive fabrication, known as stereolithography,solid objects are created by successively forming thin layers of acurable polymer resin, typically first onto a build platform and thenone on top of another. Exposure to actinic radiation cures a thin layerof liquid resin, which causes it to harden and adhere to previouslycured layers or to the bottom surface of the build platform.

SUMMARY

The present application relates generally to systems and methods forprocess optimization within an additive fabrication (e.g., 3-dimensionalprinting) device.

According to some aspects, a method of additive fabrication is providedwherein a plurality of layers of material are formed on a surface of abuild platform, each layer of material being formed so as to contact acontainer in addition to the surface of the build platform and/or apreviously formed layer of material. The method comprises forming alayer of material in contact with the container and in contact with apreviously formed layer of material, and subsequent to the forming ofthe layer of the material, separating the layer of material from thecontainer by simultaneously applying a first force to the layer of thematerial in a direction perpendicular to the surface of the buildplatform, and a second force to the layer of the material in a directionparallel to the surface of the build platform. Magnitudes of the firstforce and second force may be calculated, by at least one processor,based at least in part on one or more of a degree to which the layer ofthe material contains overhangs. The method may further comprise movinga wiper across the surface of the container.

The foregoing is a non-limiting summary of the invention, which isdefined by the attached claims.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIGS. 1A-1B depict an illustrative additive fabrication system,according to some embodiments;

FIGS. 2A-2H depict various stages during between formation of twoconsecutive layers of an object by an illustrative additive fabricationsystem, according to some embodiments;

FIG. 3 is a flowchart of a method of process optimization for additivefabrication, according to some embodiments; and

FIGS. 4A-4B are schematics showing two different views of anillustrative stereolithographic printer on which aspects of theinvention may be implemented, according to some embodiments.

DETAILED DESCRIPTION

As discussed above, some additive fabrication techniques form solidobjects by solidifying (also referred to herein as “curing”) a liquid,such as a photopolymer resin. In some such devices, a layer of solidmaterial is formed such that it adheres to either a previously formedlayer or a substrate upon which the object is built, in addition toanother surface, such as a container. In such devices, numerous problemscan arise from the application of force necessary to separate the layerfrom the surface (e.g., container) while retaining adhesion of theobject being fabricated to the substrate. It is generally inherent inseparation mechanisms that force is applied to, and through, the objectitself, yet this application of force can result in several potentialproblems.

In some instances, force(s) applied to the object being fabricated maycause the object to separate partially or totally from the buildplatform, rather than the container, thus disrupting the formationprocess. In some instances, the force applied to the object may causedeformation and/or mechanical failure in the object itself. Irrespectiveof which one or more of these potential problems might arise, it may bedesirable to minimize the separation force(s) during separation of theobject from the surface. While a relatively higher force can provideseparation more quickly, it can produce a comparatively higher risk ofdeformation in the fabricated object, whereas a relatively lower force,applied over a longer period of time, will typically furnish a moreprecisely fabricated object. One potential approach is to progressivelyseparate the object and the container, such as in a peeling operation.While this solution has a number of advantages, the process can stillintroduce unwanted forces during the separation process.

Moreover, sometimes during additive fabrication a region of liquid maybe cured to a solid or partially solid state but fail to adhere to theobject being fabricated. This may occur for a variety of reasons, suchas an incompletely performed curing operation and/or adhesion of curedor partially cured material to a surface other than the object beingfabricated (e.g., to a container). When this occurs, the cured orpartially cured material may subsequently interfere with the successfulforming of a remainder of the object, such as by adhering to anundesired location on the object and/or by inhibiting the correctformation of subsequent material (e.g., by blocking the path of aradiation source, by mechanically sticking to the object and therebyintroducing undesirable forces, etc.).

An additional problem that can occur during additive fabrication isincomplete curing of resin that does adhere to the object beingfabricated. That is, a fabricated layer of the object may include one ormore regions in which the resin is partially cured. As used herein,“partially cured” refers to material that has been cured to some extentby actinic radiation yet is chemically able to undergo additional curingto form resin with greater structural stability and/or strength. In somecases, partially cured resin can be further cured by application ofadditional actinic radiation. In some cases, partially cured resin canbe further cured simply by waiting for chemical reactions alreadyinitiated by the actinic radiation to progress further withoutnecessarily providing additional actinic radiation.

Partially cured resin can lead to structural instability, imprecisereproduction of the desired object, and/or fabrication failure. Theinventors have recognized that such problems are typically produced as aresult of either non-homogeneities and/or motion of the photopolymerresin, and/or by insufficient curing of portions of the resin.

In view of the above-described problems, the inventors have recognizedand appreciated an optimized additive fabrication process that mitigatesone or more of these problems. In particular, the optimized processincludes a number of operations that each mitigate one or more of theabove-described problems. In some embodiments, the operations may beadapted based on the geometry (shape, size, etc.) of the object beingprinted so as to maximize the quality of the particular fabricatedobject being produced.

The optimized process described herein includes a number of operationsthat may be performed between successive applications of actinicradiation to cure photopolymer resin as described above. To provide aninitial overview of the curing process, an illustrative additivefabrication system is depicted in FIGS. 1A-1B. Illustrativestereolithographic printer 100 forms an object in a downward facingdirection on a build platform such that layers of the part are formed incontact with a surface of a container in addition to a previously curedlayer or the build platform. In the example of FIGS. 1A-1B,stereolithographic printer 100 comprises build platform 104, container106 and liquid resin 110. A downward facing build platform 104 opposesthe floor of container 106, which contains a photopolymer resin 110.FIG. 1A represents a configuration of stereolithographic printer 100prior to formation of any layers of an object on build platform 104.

As shown in FIG. 1B, an object 112 may be formed layerwise, with theinitial layer attached to the build platform 104. In FIG. 1B, the layersof the part 112 are each formed from the same material but are shown inalternating colors merely to visually distinguish them in this example.The container's base surface may be transparent to actinic radiation,such that radiation can be targeted at portions of the thin layer ofliquid photocurable resin resting on the base surface of the container.Exposure to actinic radiation 115 cures a thin layer of the liquidresin, which causes it to harden. The layer 114 is at least partially incontact with both a previously formed layer and the surface of thecontainer 106 when it is formed. The top side of the cured resin layertypically bonds to either the bottom surface of the build platform 104or with the previously cured resin layer in addition to the transparentfloor of the container. In order to form additional layers of the partsubsequent to the formation of layer 114, any bonding that occursbetween the transparent floor of the container and the layer must bebroken. For example, one or more portions of the surface (or the entiresurface) of layer 114 may adhere to the container such that the adhesionmust be removed prior to formation of a subsequent layer.

As discussed above, a number of problems can potentially occur duringthis process, including incomplete curing of resin attached to the part,incomplete or complete curing of resin that remains in the container(adhered to the container and/or floating) and damage to the structureof the part during the peel process. The inventors have recognized andappreciated a number of operations that together mitigate at least theseproblems, which are described in the illustrative process of FIGS.2A-2H.

FIGS. 2A-2H depict various stages during between formation of twoconsecutive layers of an object by an illustrative additive fabricationsystem, according to some embodiments. FIGS. 2A-2H depict illustrativestereolithographic printer 200, which includes build platform 204,container 206, liquid resin 210 and wiper 203. The wiper 203 isconfigured to move along the horizontal axis of motion parallel to thebase of the container 206. In some cases, the wiper is removably mountedonto a support base beneath the container 206 and to which the containeris attached.

FIG. 2A depicts the illustrative stereolithographic printer immediatelyafter application of actinic radiation which causes a portion of theliquid resin to cure and form layer 214 of the part 212 beingfabricated. According to some embodiments, it may be advantageous todelay subsequent steps following exposure to the actinic radiation inorder to provide time for chemical reactions initiated by said exposureto occur and/or to complete. This delay is referred to herein as a “curedelay,” and can mitigate the above-described problem of partially curedresin, which can reduce the structural integrity of an object beingfabricated.

As an illustrative example, photopolymer resins such as methacrylateradical-cured photopolymer resin may substantially increase instructural stability and strength in 0.5 seconds to 1.5 seconds afterexposure to actinic radiation that initiates polymerization. As such,waiting for a cure delay period of between 0.5-1.5 seconds performedimmediately after cessation of actinic radiation to the object can beperformed to increase the quality of the object being fabricated. Othercure delay time periods may also be suitable, such as between 0.2 s-2.0s, between 0.5 s-1.0 s, between 0.75 s-1.25 s, or between 0.8 s-1.2s.

According to some embodiments, the optimal length of a cure delay periodmay be determined based upon one or more of a number of factors,including the energy level of the actinic radiation, the chemistry ofthe photopolymer resin, the temperature of the photopolymer resin, andthe geometry of the newly formed layer 214. Cure delays, such as delaysof between 0.5 seconds and 1 second, may provide a sufficient time forphotopolymer reactions initiated by actinic radiation to inducesufficient material strength to avoid unwanted distortion and/ormechanical failures in subsequent steps. In some embodiments, theoptimal delay period may be calculated based upon whether particularregions of the newly formed layer have completed fabrication or not.

As an example, it may be advantageous in certain embodiments whendetermining the length of a cure delay to distinguish between exposureof the perimeters of the newly formed layer, typically corresponding tothe outside of the object, and the interior area, sometimes known as the“fill,” or “infill.” For example, the perimeter of a layer may beexposed prior to the exposure of the fill and an optimal cure delayperiod determined based upon the completion of said perimeter exposure,rather than based upon the completion of said fill exposure, where thecure delay is timed to begin when the perimeter exposure has beencompleted. As may be appreciated, such perimeter exposures may be moresensitive to unwanted distortion due to cosmetic concerns.

In FIG. 2B, the cure delay period has ended and an operation to separatethe object 212 from the container 206 is initiated. The application offorce to the object to separate it from the container may be necessarydue to adhesive and/or fluid forces. For instance, the photopolymer mayhave adhered to the container during the curing process and/or theabsence of liquid between the cured photopolymer and the container mayproduce suction forces acting against separation of the two.Irrespective of the source of the attachment between the object and thecontainer, such attachment must be broken in order to continue theformation process. This attachment can be broken using varioustechniques, including by horizontally sliding the container, by rotatingthe container about an axis (e.g., at the edge of the container), bymoving the container downwards away from the build platform, or bycombinations thereof.

In some embodiments, attachment between the object and container may bereduced by applying a force to container 206 along an x-axis, shown asforce 231 in FIG. 2B). In some cases, the container 206 may be coupledto a supporting structure (e.g., beneath the container) that is moved inthe x-direction, thereby producing the force 231 in the x-direction. Inaddition, or as an alternative, such attachment may be reduced byapplying a force to the object 212 through the z-axis, shown as force232 in FIG. 2B. The force 232 may be produced by translating the buildplatform 204 up along the z-axis, and/or by moving the container 206down along the z-axis.

According to some embodiments, the system may apply forces to thecontainer in both the x-axis and z-axis directions in order toeffectively separate the object from the container. In particular, theinventors have determined that it may be advantageous to apply a forcein the z-axis direction, referred to herein as a “preload force,” priorto initiating application of a force along the x-axis. In many cases,the z-axis force may not result in actual movement of the build platformrelative to the bottom of the container due to the attachment betweenthe newly formed layer 214 and the bottom of the container. The preloadforce, however, may instead introduce forces that begin to overcome suchattachment.

As one example, the inventors have recognized that a desirable magnitudeof the preload force may be calculated based on the effective surfacearea of the layer just fabricated (e.g., the area of layer 214 facingthe container 206 in FIG. 2B). In some cases, the magnitude of thepreload force may be determined by interpolating based uponpredetermined effective area calculations. For example, values of theideal preload force may be predetermined for two or more correspondingvalues of the layer area size. During fabrication, the additivefabrication system may determine the layer area of the layer justfabricated and perform linear interpolation between the two nearestlayer area values and their corresponding preload force values todynamically determine a suitable preload force to apply. The calculatedforce can then be applied through the z-axis as discussed above.

In some cases, it may be advantageous to dynamically determine amagnitude of the preload force by reference to previously calculatedvalues for given ranges of effective areas. Said values may beexperimentally determined so as to ensure successful layer formation.The inventors have recognized that geometrical features of thejust-fabricated layer may be advantageously considered in determiningsaid values. As one example, the optimal magnitude of the preload forcemay be calculated based on the ratio of the area to the perimeter of alayer just fabricated.

According to some embodiments, separation forces may be simultaneouslyapplied in both the x- and z-directions. Such forces may be appliedsubsequent to a preload force applied in the z-direction, should apreload force be applied as described above. The combined x- andz-direction forces may separate the object 212 from the container, thencause the object to move to a predetermined position relative to thecontainer. The velocity and distance of such motions may be set toconstant values, determined experimentally, to ensure successful layerformation. Since the x- and z-direction forces may have variability inhow much force is necessary to separate the objects, the amount of timeduring which the forces are applied prior to separation may vary fromlayer to layer during fabrication.

According to some embodiments, the absolute and relative magnitudes offorces applied in the x- and z-directions may be dynamically determinedbased upon numerous factors, including the geometry of the layer justfabricated, the geometry of one or more prior fabricated layers (e.g.,the second-to-previous layer fabricated, etc.), the height of the buildplatform above the bottom of the container, and/or the viscosity of thephotopolymer resin. The determined forces may be applied simultaneouslyin the separation operation.

As an example, the inventors have recognized that the optimal magnitudeof the force applied in the x-direction may be inversely proportional tothe length of the longest extent of the just-fabricated layer of theobject measured in the x-axis. In some embodiments, therefore, theadditive fabrication system can determine the extent of thejust-fabricated layer of the object along the x-axis and calculate amagnitude of a force to apply along the x-direction during separation.Since layers of an object will generally have a different size and shapethrough a fabrication process, x-axis forces determined in this mannerwill therefore generally change in value from layer to layer.

In some implementations, the separation force applied along the x-axismay be calculated based on the length of the layer in the x-axis asdescribed above yet further modified when the length is above somecutoff value. For example, for lengths less than a critical value, suchas the expected distance that the container will move in the x-directionas a result of application of the x-direction force (or this distancemultiplied by a factor between 0.9 and 1.1), the magnitude of thex-direction force may be modified. However, for x-axis lengths greaterthan this critical value, the x-axis force calculated as described abovemay be reduced by k, wherein k is experimentally determined based uponthe configuration of the system to ensure reliable part formation. Ingeneral, optimal values of k may be found by increasing k (therebyreducing the force applied) until print failures occur, such as thosecaused by failures to detach due to inadequate force exerted in thex-axis direction. In some embodiments, it may be preferred to optimizefor print success, rather than for minimal forces, and thus in somecases an optimal value of k may be determined by increasing k until theforces exerted in the x-axis direction are above the threshold wherepart separation failures are observed.

The inventors have also recognized that the optimal magnitudes ofseparation forces including the preload force and/or subsequent forcesin the x- and/or z-directions may depend upon the viscosity of thephotopolymer resin used in the additive fabrication device. In someembodiments, therefore, each of the preceding force magnitudecalculations may be further based upon a standardized viscositymeasurement V. In such embodiments, said force magnitudes may becalculated by multiplying the result achieved via the above procedure(s)multiplied by a factor of the actual (working) viscosity of thephotopolymer divided by the standardized viscosity measurement resin tobe used. As may be appreciated, the actual viscosity of the photopolymerresin may be determined experimentally for a given operating temperatureor determined experimentally for a reference temperature and adjustedbased upon the actual temperature of the photopolymer resin.

According to some embodiments, magnitudes of separation forces includingthe preload force and/or subsequent forces in the x- and/or z-directionsmay be determined based on a degree to which a given layer of the objectcontains overhangs, referred to herein as an overhang degree. Anoverhang is an area of a layer that does not attach to either the buildplatform or to a layer formed immediately prior. Overhangs may affecthow the object is able to withstand forces applied during the separationprocess due, in part, to the fact that areas of overhang may be moresusceptible to deformation, depending on the mechanical strength of thelayer areas forming the overhang. Different shapes of overhangs may havediffering mechanical strengths. In some cases, for example, an overhangarea may extend out from non-overhanging areas, and thus be supported ononly one edge of the overhang. In other cases, however, an overhangingarea may extend from more than one non-overhanging area such that morethan one edge of the overhang is supported. As will be appreciated, suchoverhangs share many mechanical properties with a bridge, and are infact sometimes known referred to as bridges. Various heuristics may beutilized to characterize an overhang degree.

In some implementations, the overhang degree in a given layer N ischaracterized by the area of the Boolean difference between the currentlayer (N) and the previous layer (N−1). In some implementations,overhang degree is characterized by the maximum distance from anoverhanging point to a non-overhanging point. In some implementations,an overhang degree may be determined by a structural analysis ofoverhanging areas, such as finite element analysis (FEA), based on theexpected maximum allowable loading on overhang structures. An example ofsuch a FEA technique may be found in U.S. patent application Ser. No.14/543,138, titled “Systems and Methods of Simulating Intermediate Formsfor Additive Fabrication,” filed on Nov. 17, 2014, and which is herebyincorporated by reference in its entirety.

In some embodiments, an overhang index may be calculated in thefollowing manner: first, the area of all overhang portions, asdetermined based on Boolean comparison with the proceeding layer, isdetermined; next, overhanging areas of regions with an area greater thansome cutoff value (e.g., 0.5 mm²) are summed together and the squareroot of such areas are determined; finally, the overhang index isprepared by mapping said square root value to a linear scale of 0.0 to1.0, wherein 0.0 represents an area square root value of 0 and 1.0represents area square root values of more than 5 mm. Magnitudes ofseparation forces including the preload force and/or subsequent forcesin the x- and/or z-directions may be determined based on the overhangindex determined via this process. For example, in some implementations,calculation of one or more such separation forces may be based on afactor of ⅓^(I), where I is the overhang index having a value between 0and 1 (e.g., the force may be equal to a calculated value multiplied bythis factor).

In some embodiments, magnitudes of separation forces including thepreload force and/or subsequent forces in the x- and/or z-directions maybe determined based on a degree to which a given layer of cured resinforms a portion of a feature or area of an object that is substantiallymore fragile than other portions of the object, referred to herein as afragility degree. The fragility degree may vary between layers,depending on, among other factors, the geometry of a given layer. Insome embodiments, a fragility degree may be ascertained by performing astructural analysis and/or finite element analysis of the object todetermine which regions have a lower resistance to separation forcescompared with the average resistance of the object being fabricated.

In some implementations, the fragility degree may be determined based onthe extent to which a layer contains isolated regions of surface areabelow a threshold value. In some implementations, a fragility degree mayalso be determined based upon the extent to which a layer containsregions of high aspect ratio in their dimensions. In someimplementations, a fragility degree may be characterized by a ratio oflayer surface area to layer perimeter length.

With respect to FIG. 2B, the proceeding examples have been describedwith reference to determinations made based on the layer 214 currentlybeing deposited and separated from the container 206. In some cases,however, it may be advantageous when calculating one or more separationforces to also consider similar properties in layers formed before orafter the current layer. As one example, the inventors have noted thelayers previously formed may restrict the flow of liquid resin such thatthe forces involved during separation tend to be increased. In this way,previously formed layers of high surface areas may tend to influence thecurrent layer and increase an effective adhesion index (a measure ofadhesive forces) for the current layer. In testing, the inventors havefound that for a resin with viscosity on the order of 800 cP, propertiesof layers located up to 2 mm above the last fabricated layer may beconsidered when calculating indexes such as adhesion index, overhangindex, and fragility index, among others.

According to some embodiments, magnitudes of separation forces includingthe preload force and/or subsequent forces in the x- and/or z-directionsmay be determined based on a degree to which the useful life of a resincontainer has been expended, referred to herein as a wear degree. Someresin containers may be constructed with materials chosen for their lowreactivity and adhesion coefficients. Over time, however, the materialproperties of many resin container designs tend to change, due in partto the mechanical and chemical stresses incurred when each layer isformed and removed from the resin container floor. Such changes inmaterial properties typically result in increased object failure ratesdue to an increase in the adhesive forces. The wear degree characterizesthe extent to which a given build tray may have suffered suchdegradation. The wear degree may vary between prints as well as betweenlayers, as the build tray's useful life decreases with each layer formedand removed.

Numerous heuristics may be considered to characterize the wear degree.As one example, the inventors have observed that the wear degree of acontainer may be related to the aggregate number of layers fabricatedusing that container. In some cases, the wear degree may be calculatedfrom a linear or nonlinear function of the aggregate number of layersand/or of the expected average maximum number of layers for a givenresin container design. In addition, it may be advantageous to track thewear degree for a given resin container for different regions of theresin container, given the tendency for layer formation to beconcentrated at the center of a resin container.

Any of the above-described techniques in which various characteristicsof the additive fabrication device and/or object being fabricated areconsidered when calculating the magnitude of one or more separationforces may be combined and used together. As described above,consideration of various characteristics may be performed in practiceusing a factor calculated through the above-described analysis. It willbe appreciated that separation forces can be calculated by multiplyingtogether several such factors with an initial force value to arrive at aseparation force value that is applied via the additive fabricationdevice. Such a calculation may be performed by hardware of the additivefabrication device and/or by a computing device that providesfabrication instructions to the additive fabrication device, and may beperformed during additive fabrication and/or prior to the beginning offabrication.

Irrespective of which of the above-described techniques for calculatingseparation force(s) are followed, after the separation, object 212 maybe raised by moving build platform 204 up along the z-axis, therebyproducing the configuration shown in FIG. 2C.

In the example of FIGS. 2A-2H, after separation is at least partiallycompleted, the wiper 203 begins to move across the surface of thecontainer 206. According to some embodiments, the wiper may beconfigured to remove pieces of resin adhered to the container and notattached to the object 212 after separation. In some cases, wiper 203may be a wiper as is described in U.S. patent application Ser. No.15/248,295, titled “Techniques For Surface Preparation During AdditiveFabrication and Related Systems And Methods,” filed on Aug. 26, 2016,and which is hereby incorporated herein by reference in its entirety.

According to some embodiments, wiper 203 may be moved across the surfaceof the container and subsequently returned to its starting position, asshown in FIGS. 2C, 2D, 2E and 2F, prior to the formation of anadditional layer. In some embodiments, it may be advantageous to limitthe motion of the wiper 203, such that wiper 203 is free to travel onlywithin a region of the bottom of container 206, such as a regionexpected to be used for the formation of an additional layer of theobject.

The inventors have recognized that a wiper having a height H may begin awiping motion as soon as the object has been moved up along the Z axisby at least a distance of H. Moreover, given that the object may bemoved up along the Z axis while the wiper moves, it is further possibleto begin wiper motion before the clearance of H has been established, solong as the wiper will not reach any portion of the object being formedprior to that portion of the object reaching the clearance height of H.Indeed, in the example of FIGS. 2C-2D, the wiper 203 begins to moveprior to the object 212 reaching a sufficient height to clear the motionof the wiper (the object reaches such a height in FIG. 2D).

According to some embodiments, a time at which to being motion of thewiper may be determined using a suitable collision detection technique.In some implementations, for example, a bounding box of the union of alllayers in the object within a distance H from the bottom of the objectmay be calculated and used to check for potential collisions between aplanned wiper motion along the x-axis and a planned motion of the buildplatform and object along the z-axis. Moreover, the same technique maybe utilized, in reverse, in order to move the wiper across the bottom ofthe container while the build platform and object are moved down, suchas in a squish operation to be described below.

Subsequent to completion of the wipe operation, the build platform 204may be lowered along the z-axis into position for the formation of a newlayer onto object 212. During this stage, referred to herein as a“squish” stage, photopolymer resin located between the bottom of object212 and the bottom of the container 206 may be pushed out as the objectdescends, finally resulting in a layer of photopolymer resin of adesired thickness between the bottom of the object 212 and the bottom ofthe container 206. The end of such a squish motion is shown in FIG. 2G,with region 220 being a region of photopolymer resin to be targeted forcuring in a subsequent curing operation.

In some embodiments, it may be advantageous to delay for a period, knownas a “post-squish wait,” before subsequently exposing the region ofphotopolymer resin 220 to actinic radiation. In particular, theinventors have recognized and appreciated that immediately after thesquish operation, at least some of the liquid resin the container 206may be in motion. To begin curing of the resin without a wait maytherefore risk curing of regions of resin whilst those regions are inmotion. This can reduce the accuracy of the object being fabricatedbecause the cured portions may not finish curing in their intendedlocation. Furthermore, such effects may negatively impact the structuralintegrity of the object.

According to some embodiments, a post-squish wait period may be set to asufficiently high constant value, determined experimentally, to ensuresuccessful layer formation. As an example, a post-squish wait of between0.1-0.5 seconds after the completion of the squish motion may allowvarious components in the system to reach an equilibrium state under thevarious static and fluid forces applied.

In some embodiments, post-squish wait periods may be dynamicallydetermined based upon numerous factors, including the geometry of thecurrent layer, the geometry of prior layers, the height of the buildplatform above the bottom of the container, and the viscosity of thephotopolymer resin used. As one example, a greater post-squish waitperiod may be selected when it is determined that the surface area ofthe layer about to be fabricated and/or the surface area of any previouslayer within a distance of 2 mm from the layer about to be fabricated isabove an average surface area for the layers of the object.

In some cases, motion of the surface of the build platform through thephotopolymer resin may influence how the resin spreads out in thecontainer both during said motion and for a time after the buildplatform stops moving. As such, when the lower surface of the buildplatform is in contact with the photopolymer resin, it may beadvantageous to consider the surface area of the build platform as if itwere one or more prior layers of the part for the purposes ofcalculating a post-squish wait. Additionally, as the viscosity of thephotopolymer resin used increases, it may be advantageous to increasethe post-squish wait in order to allow the liquid resin to settle into auniform thickness in the container. As such, the post-squish wait may bedetermined based on a viscosity of the resin.

As one example, the inventors have recognized that it is desirable tocalculate the post-squish delay based on the effective area of the layerabout to be formed immediately after the post-squish wait ends. In someembodiments, this recognition leads to the following optimizationstrategy: A maximum squish delay time may be experimentally determinedfor the full area of the build platform. The effective area of the layerto be formed may then be determined and the maximum post-squish delaydetermined by linear interpolation, wherein a zero-area cross sectionhas no post-squish delay and the full area of the build platformreceives the maximum post-squish delay. In some embodiments, saideffective area may be simply determined by geometric analysis of thelayer to be formed. In other embodiments, however, it may beadvantageous to consider the geometry of previously formed layers, aswell as the build platform, so as to account for their influence in thefluid forces exerted during a squish operation. In some embodiments,such influences may be accounted for by calculating the effective areaof the layer to be formed by summing the area of the layer to be formedwith the effective area of each previously formed layer, each multipliedby a factor representing the extent to which the previously formed layeris calculated to influence the squish operation. This factor may bedetermined experimentally by dividing the observed flow coefficient byresin viscosity. In some embodiments, a factor of 0.75-0.85 per 100 μmin layer height has been found to be effective. And, as will beappreciated, post-squish wait calculated as provided above may befurther optimized based on additional parameters.

Once the squish wait operation completes, actinic radiation may beapplied to form a new layer 215, as shown in FIG. 2H. The process ofFIGS. 2A-2H may then be repeated for each layer of the object.

FIG. 3 is a flowchart of a method of process optimization for additivefabrication, according to some embodiments. Method 300 performs thesequence of operations described above in relation to FIGS. 1B and2A-2H.

In act 302, a layer of an object may be formed via application ofactinic radiation as shown in FIG. 1B and FIG. 2A. In act 304, a curedelay operation may be performed as shown in FIG. 2A. In act 306, aseparation operation may be performed as shown in FIG. 2C. In act 308, awipe operation may be performed as shown in FIGS. 2D-2F. In act 310, asquish operation may be performed as shown in FIG. 2G. In act 312, asquish wait operation may be performed as shown in FIG. 2G. After act312, method 300 returns to act 302 to produce another layer of theobject as shown in FIG. 2H.

It will be appreciated that not all of acts 304, 308, 310 and 312 may beperformed in every period between successive applications of actinicradiation, although each such period may potentially include one or moreof these acts. For example, a cure delay operation may be performedafter the application of actinic radiation only for some layers; e.g.,the additive fabrication device may be configured to not perform a curedelay when the just-fabricated layer is smaller than some thresholdsize. Thus optimization of speed of the additive fabrication device mayinclude a determination of which of the above-described operations aredesirable, or not, to produce an object with a desired accuracy.Furthermore, each application of an operation in method 300 may beapplied differently at different times. For example, forces appliedduring successive separation operations may be different due to thefactors considered in calculating those forces, as discussed above.

Furthermore, in some cases, certain acts of method 300 may be executedin parallel. For example, as discussed above, the wiper operation maybegin in at least some cases before the separation operation hascompleted. Similarly, the squish operation may begin, in at least somecases, before the wipe operation has completed.

FIGS. 4A-4B are schematics showing two different views of anillustrative stereolithographic printer on which aspects of theinvention may be implemented, according to some embodiments.

Illustrative stereolithographic printer 400 comprises a support base401, a display and control panel 408, and a reservoir and dispensingsystem for photopolymer resin 404. The support base 401 may containvarious mechanical, optical, electrical, and electronic components thatmay be operable to fabricate objects using the system. During operation,photopolymer resin may be dispensed from the dispensing system 404 intocontainer 402. Build platform 405 may be positioned along the verticalaxis 403 such that the bottom facing layer of an object beingfabricated, or the bottom facing layer of build platform 405 itself, isa desired distance from the bottom 411 of container 402. The bottom 411of the container 402 may be advantageously transparent to actinicradiation generated by a source located within the support base (notshown) such that liquid photopolymer resin located between the bottom411 of container 402 and the bottom facing portion of build platform 405or an object being fabricated thereon, may be exposed to the radiation.Upon exposure to such actinic radiation, the liquid photopolymer may becured and attached to the bottom facing portion of build platform 405 orto an object being fabricated thereon. (FIGS. 4A-B represent aconfiguration of stereolithographic printer 401 prior to formation ofany layers of an object on build platform 405.) A wiper 406 isadditionally provided, capable of motion along the horizontal axis ofmotion 410 and which may be removably mounted onto the support base at409. The wiper may be coupled to one or more actuators (e.g., steppermotors, belts attached to motor(s), etc.) that produce lateral movementof the wiper across the surface of the container.

As discussed above, a number of the optimization steps described mayhave one or more parameters that can be calculated based on one or moreof the described factors, such as qualitative aspects of the geometry ofthe object being fabricated. These parameters can be calculated by atleast one processor during fabrication, before fabrication, or both(i.e., at least some aspects of a given calculation may be performedpre-fabrication with others performed during fabrication.). In thedescription above, where an “additive fabrication system” is referencedas performing calculations, this description is intended to encompassboth an additive fabrication device that includes one or more processorsas well as additive fabrication device coupled to an external computingdevice, as the particular location at which the calculations areperformed is not limited only to the device executing the fabricationprocess.

As such, the at least one processor calculating one or more parametersfor one or more of the optimization steps may be part of the additivefabrication device itself and/or may be located in a computing devicecoupled via a wired and/or wireless connection to the additivefabrication device. This coupling may be temporary in nature—forexample, the processor of a computing device may calculate parametervalue(s) for use in an optimization step of the above-described processand wirelessly transmit that value to an additive fabrication device,which stores the value(s) and retrieves them during later fabrication.In some cases, parameters stored by an additive fabrication device maybe modified during fabrication based on various aspects of thefabrication process as described above. For example, a “baseline” valuefor a parameter may be stored and a multiplicative factor applied to theparameter value during each layer fabrication process to producevariations in the way the parameter is applied during fabrication.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A method of additive fabrication wherein aplurality of layers of material are formed on a surface of a buildplatform, each layer of material being formed so as to contact acontainer in addition to the surface of the build platform and/or apreviously formed layer of material, the method comprising: forming alayer of material, the layer of material being in contact with thecontainer and in contact with a previously formed layer of material;subsequent to the forming of the layer of the material, separating thelayer of material from the container by simultaneously applying: a firstforce to the layer of the material in a direction perpendicular to thesurface of the build platform; and a second force to the layer of thematerial in a direction parallel to the surface of the build platform,wherein magnitudes of the first force and second force are calculated,by at least one processor, based at least in part on a degree to whichthe layer of the material contains overhangs; and moving a wiper acrossthe surface of the container.
 2. The method of claim 1, wherein thefirst force is calculated, by the at least one processor, based at leastin part on a length of the longest extent of the layer of the materialin the direction perpendicular to the surface of the build platform. 3.The method of claim 1, wherein the layer of material is formed from aliquid photopolymer and wherein the first force and/or the second forceis calculated, by the at least one processor, based at least in part ona viscosity of the liquid photopolymer.
 4. The method of claim 1,wherein the first force and/or the second force is calculated, by the atleast one processor, based at least in part on a measure of fragility ofat least a portion of the layer of material.
 5. The method of claim 1,further comprising applying a preload force in the directionperpendicular to the surface of the build platform prior to applicationof the first force and second force.
 6. The method of claim 5, furthercomprising calculating, by the at least one processor, the preload forcebased at least in part on an area of the layer of material.
 7. Themethod of claim 1, further comprising waiting for a predetermined curetime subsequent to the forming of the layer of the material and prior toactively separating the layer of material from the container.
 8. Themethod of claim 7, further comprising calculating, by the at least oneprocessor, the cure time based at least in part on an area of the layerof material.
 9. The method of claim 1, wherein the layer of material isa first layer of material, and wherein the method further comprises:waiting for a predetermined wait time subsequent to moving the wiperacross the surface of the container; and after the predetermined waittime, forming a second layer of material, the second layer of materialbeing in contact with the first layer of material.
 10. The method ofclaim 9, further comprising calculating, by the at least one processor,the wait time based at least in part on an area of the first layer ofmaterial.
 11. The method of claim 1, wherein the first force is appliedby moving the container using at least one actuator.
 12. The method ofclaim 1, wherein the second force is applied by moving the buildplatform away from the container using at least one actuator.
 13. Themethod of claim 1, wherein the wiper comprises a wiper arm and a wiperblade coupled to said wiper arm using a pivoting coupling.
 14. Themethod of claim 1, wherein the moving of the wiper across the surface ofthe container begins during application of the first and second forcesand ends after application of the first and second forces has completed.